The present invention relates to a distance measuring device for an automatic focusing camera. More particularly, the invention relates to a distance measuring device of a triangulation system type for measuring a distance to each of at least two or more different points of an object which appears in a predetermined field of view of the camera.
Recently, automatic focusing (AF) cameras, which are able to measure a distance to an object and then to effect focusing of the object by automatically moving a photographing lens on the basis of the result of distance measurement, have been widely developed and used.
Generally, conventional AF cameras are so constructed as to measure a distance to an object which appears in the center of a field of view of a finder of the AF camera, and then to automatically move a photographing lens on the basis of the result of distance measurement. Accordingly, an undesirable mis-focusing phenomenon, such as a so-called center passing phenomenon, tends to occur in the conventional AF cameras. A typical example of such an undesirable phenomenon will be explained below in more detail.
When the AF camera is used in a state that an object, such as a person, which is to be focused, appears out of a central area mark provided in the finder, while background existing substantially in infinity appears in the central area mark, the background will be brought into focus, with the object being out of focus, because the photographing lens is adjusted in accordance with the result of measurement of a distance to the background.
Various AF mechanisms for preventing the above-described undesirable phenomenon have been proposed.
For example, Japanese Patent Application Laying-Open No. 59-193307 (referred to as a first reference hereinafter) discloses an AF mechanism for a camera, which comprises a distance measuring device for measuring a distance to each of a plurality of different points, one appearing in the central area of a field of view of a finder of the camera and others appearing out of the central area of the field of view. In this AF mechanism, a movable part of the photographing lens is adjusted in position in accordance with the shortest distance selected from the plurality of distances each obtained by the measurement.
Japanese Patent Application Laying-Open No. 62-223734 (referred to as a second reference hereinafter) also discloses a distance measuring device for an AF camera, which can measure a distance to each of a plurality of different points within a field of view of a finder of the camera. The distance measuring device comprises a light emitting element consisting of an array of three light emitting diodes, and a large-sized position sensitive device as a distance measuring element. In order to prevent the above-described center-passing phenomenon, the device is adapted to measure a distance to each of three different points by using different three optical axes for measurement which are slanted with respect to each other and one of which extends substantially in parallel to an optical axis of a photographing lens.
Further, Japanese Patent Application Laying-Open No. 57-197414 (referred to as a third reference hereinafter) also discloses a distance measuring device for an AF camera, which comprises a pair of optical systems for imaging an object, which are spaced from one another with an interval corresponding to a predetermined reference length. The distance measuring device further comprises a pair of photosensor-arrays, such as charge-coupled device (CCD), which are placed on respective positions on which images of an object are formed, respectively. In the distance measuring device of the third reference, a spatial phase-discrepancy between the two images formed on the respective photosensor-arrays is detected, and a distance to the object is calculated on the basis of the result of detection of the phase discrepancy. Further, in the AF mechanism of the third reference, a component, such as a mirror, of at least one of the two optical systems is movable mechanically in stages so that a point of intersection of the optical axes of the optical systems can be shifted toward a position closer to the camera. Accordingly, a minimum measurable distance can be decreased.
In the distance measuring device disclosed in the first reference, a light emission system consisting of a light emitting element and an optical components for light emission is so controlled as to emit a light beam forwardly from the camera and scan it over a predetermined angular range from a position close to the camera to infinity, while a light receiving system consisting of a light receiving lens and a photoelectric conversion element is maintained stationarily. Accordingly, the device disclosed in the first reference has a disadvantage in that tight tolerances of the components of the device are required in machining and assembling processes thereof. Plays are tends to be generated in the light emission system and the light receiving system due to wear during the repetition of the operation of the distance measuring device, resulting in reduced accuracy of distance measurement.
Further, the distance measuring device disclosed in the second reference is advantageous with respect to a stability of accurate measurement and durability of the device, because a light emission system, which is comprised of a plurality of light emitting elements and a lens, and a light receiving system comprised of a plurality of unidimensional position-detecting elements, each of which is made of a semiconductor, and a light receiving lens, are maintained stationarily. However, the device disclosed in the second reference has a disadvantage in that the light emission system and the light receiving system are complicated in construction, and large occupational spaces for accommodating these systems are required, resulting in increased manufacturing cost.
Furthermore, the distance measuring devices disclosed in the first and second references can effect distance measurement in a multi-point measuring mode such as a three-point measuring mode, as mentioned above. However, in these devices, movement of the measuring center axis is restricted within a predetermined fixed range. Accordingly, when a lens of a variable focal point type is used as the photographing optical lens, the angle of view of the lens will be varried in accordance with changes in the focal point thereof, and thus a measuring point will be shifted on the surface of a photosensitive film. Therefore, these devices can not obtain a satisfactory result of distance measurement in the multi-point measuring mode.
On the other hand, in the distance measuring device disclosed in the third reference, the movable component of the imaging optical system has to be manufactured and assembled in a high accuracy, resulting in increased manufacturing cost. Plays are apt to be generated in the movable component due to wear thereof during the repetition of the operation of the distance measuring device, resulting in reduced accuracy of distance measurement.